Optical dish device

ABSTRACT

An information recording device having a light source unit including a plurality of laser light sources which are substantially linearly arrayed and independently driven, a deflecting unit for periodically deflecting a plurality of beams of laser light emitted from the light source unit in a direction crossing that of an array of the beams, and an image forming optical system for focusing the plurality of laser beams from the deflecting unit on a photoreceptor. In the information recording device, optical units for reducing divergence of the laser beams are provided in connection with the laser light sources of the light source unit.

This is a division of application Ser. No. 08/386,594, filed Feb. 10,1995, now U.S. Pat. No. 5,619,488 which is a continuation of Ser. No.07/941,155, filed Sep. 4, 1992, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an information recording device, suchas a digital copying machine, a laser beam printer or an optical disksystem, and more particularly to an information recording device of thetype in which a plurality of light sources, such as a multibeamsemiconductor array, are used and the surface of an object to be scannedis simultaneously scanned with a plurality of scan lines.

2.Discussion of the Related Art

In the laser beam printer, for example, a laser beam modulated by animage signal is applied to a polygon scanner revolving at a high speed.The laser beam is reflected by the scanner, and scans the surface of arecording medium, e.g., a photoreceptor, in the main scan direction, toform a latent image on the surface. The latent image is developed into atoner image. The toner image is transferred onto a sheet of recordingpaper.

In this type of laser beam printer, to increase a resolution of an imageor to reduce the time taken for the image formation, it is necessary toincrease the revolving speed of the polygon scanner. However, physicalrestrictions, such as weight of the polygon scanner and torque of adrive motor, hinders the increase of the revolving speed of the polygonscanner.

To solve the problem, a multibeam scan system in which the surface ofthe scanned object is simultaneously scanned with a plurality of beams,has been proposed and put into practice. In this scan system, as amatter of course, a plurality of beam spots must be arrayed in a mannerthat those spots are made satisfactorily close to each other in thedirection (referred to as a subsidiary scan direction) orthogonal to thescan direction (referred to as a main scan direction) of the scan by thepolygon scanner. Aggressive efforts to manufacture a plurality ofsemiconductor lasers close together have been made and now progress. Thespot-to-spot distance of 10 μm has been achieved in the semiconductorlaser arrays thus far proposed (reference is made to Japanese PatentUnexamined Publication No. Hei. 2-39583 and R. L. Thornton et al.,"Properties of Closely Spaced Independently Addressable LasersFabricated by impurity-Induced Disordering" Appl. Phys. Lett. 56(17),1623-1625 (1990)).

Also, there are optical approaches to reduce the spot-to-spot distanceof the laser beams, as disclosed in Japanese Patent UnexaminedPublication No. Sho. 54-7328, which uses an optical fiber or an opticalwaveguide for the spot-to-spot distance reduction, and Japanese PatentUnexamined Publication No. Sho. 59-15218, which uses a prism or areflecting mirror. Additionally, Japanese Patent Unexamined PublicationNo. Sho. 54-38130 effectively reduces the spot-to-spot distance in thesubsidiary scan direction by inclining a semiconductor laser array.Japanese Patent Unexamined Publication No. Sho. 56-110960 uses theinterlaced scanning method to fill the spaces each between the adjacentlaser beam spots through a plural number of scans.

An example of the spot-to-spot distance reduction method based on theinterlaced scanning is shown in FIG. 19. In this example, two laserbeams LB1 and LB2 are used for the interlaced scanning. In FIG. 19,d_(x) represents the diameter of a laser beam spot that iselectrophotographically defined. The laser beam spot diameter is thediameter of the spot of an image that is formed by using a laser beamhaving a certain light intensity distribution, and will be referred toas an electrophotographic spot diameter. The spot diameter of a laserbeam is generally defined as the diameter of the beam spot of which thecircumference is at an intensity of light 1/e (1/e² in power) as greatas the light intensity at the center of the beam spot. The spot diameterthus defined is called an optical spot diameter and denoted as d₀. Inthe description to follow, the imagery spot diameter of the laser beamwill be based on the optical spot diameter, unless otherwise noted.

A relationship between the optical spot-diameter and theelectrophotographic spot diameter is graphically illustrated in FIG. 20.A ratio of the optical spot diameter d₀ to the electrophotographic spotdiameter d_(x) is called a spot-diameter correction coefficient "k". Thespot-diameter correction coefficient "k" is mathematically defined as

    k=d.sub.0 /d.sub.x                                         ( 1)

The value of the spot-diameter correction coefficient "k" depends on theelectrophotographic process used. In the process of the charged-areadevelopment where toner is attached to an area exposed to light, thecoefficient "k" is preferably selected to be within 1.4≦k≦1.6, while inthe process of discharged area development where toner is attached to anarea not exposed to light, it is preferably selected to be within1.5≦k≦1.8.

The center-to-center distance r₃ between the two spots imaged on thesurface P0 to be scanned by the two laser beams LB1 and LB2 is given byr₃ =3d_(x). The subsidiary scan progresses by 2d_(x) every main scan.Accordingly, as shown in FIG. 19, in the first scan, the second scanline is traced by the laser beam LB2. In the second scan, the first scanline is traced by the laser beam LB1, and the fourth scan line is tracedby the laser beam LB2. Thus, a gap is formed for each scan; however, ina scan, the first laser beam of the paired ones traces a first scanline, and in the next scan, the second laser beam skips over thepreviously traced scan line and traces a second scan line. In this way,the scan lines are successively traced in a gapless fashion.

If the already-described semiconductor laser array having the lightemitting points closely arrayed at 10 μm spatial intervals (JapanesePatent Unexamined Publication No. Hei. 2-39583), is operated in theinterlaced scanning mode as shown in FIG. 19, the resultant multibeamlaser beam printer, in principle, would be high in definition andoperating speed.

However, it is very difficult to actually manufacture the semiconductorlaser array having the light emitting spots arrayed at the 10μm-intervals for the multibeam laser beam printer, for the followingreasons. When the adjacent light emitting spots or semiconductor laserelements are spaced by 10 μm, the thermal crosstalk between the adjacentelements becomes problematic. It was confirmed that to reduce thethermal crosstalk to such a value as to be practically negligible, theoscillation threshold value of each semiconductor laser element must bereduced to approximately 10 mA. In the case of a semiconductor laser ofAlGaAs, which oscillates at approximately 780 nm in the spectrum of theinfrared rays, the semiconductor laser element of such a low oscillationthreshold value can be manufactured by using the technique at thepresent stage. In the case of a semiconductor laser of AlGaInP, whichoscillates at further shorter wavelength of 680 nm, the laser elementsthat can be manufactured are only those each oscillating at anoscillation threshold value several times as large as that of the laserelement of the AlGaAs laser.

The laser beam printer employs the electrophotographic process. Theelectrophotography was developed, at the beginning, for the copyingmachine in which the photoreceptor is exposed directly to lightreflected from an original document. Some of the photoreceptorsspecially designed for the laser beam printer are sensitive to theinfrared rays of approximately 780 nm. Such photoreceptors areunsatisfactory in lifetime and reliability performances. Thephotoreceptors sensitive to the infrared rays are not required for theusual copying machine. If the wavelength of laser light emitted from asemiconductor laser light source could be confined within the visiblespectrum, the photoreceptor may be used for both the laser beam printerand the normal copying machine. This leads to cost reduction. Also in acase where the AlGaAs semiconductor laser oscillating at approximately780 nm is used, when a large optical power is required for a high speedprinter, a large current must be fed when it is driven, if only thethreshold value is reduced. Also in this case, the thermal crosstalkproblem also arises.

As described above, it is desirable to manufacture a semiconductor laserarray using the AlGaInP semiconductor laser oscillating at approximately680 nm in the visible spectrum. However, it is difficult to manufacture,by the present technique, a semiconductor laser array, which consists ofthe laser elements arrayed at 10 μm intervals, oscillates atapproximately 680 nm, and has a satisfactorily suppressed thermalcrosstalk. Also in a case where the semiconductor laser having a lowthreshold value and oscillating at approximately 780 μm, is used, it isdifficult to manufacture a large power semiconductor laser array, whichconsists of semiconductor elements spaced by 10 μm and has asatisfactorily suppressed thermal crosstalk.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstancesand has an object to provide an information recording device which canreduce the distance between imagery spots by laser beams on aphotoreceptor without reducing the distance between semiconductor laserelements of a semiconductor laser array.

In order to attain the above object, the present invention provides aninformation recording device including: light source means including aplurality of laser light sources which are substantially linearlyarrayed and independently driven; deflecting means for periodicallydeflecting a plurality of beams of laser light emitted from the lightsource means in a direction crossing that of an array of the beams; animage forming optical system for focusing the plurality of laser beamsfrom the deflecting means on a photoreceptor; and optical means forreducing divergence of the laser beams in connection with the laserlight sources of the light source means.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification illustrate embodiments of the invention and,together with the description, serve to explain the objects, advantagesand principles of the invention. In the drawings,

FIG. 1 is a view showing the construction of information recordingdevice of the invention, in which two laser light sources arerespectively followed by two lenses;

FIG. 2 is a view showing an optical system in which two laser lightsources are followed by a single lens;

FIG. 3 is a view showing a model of an image forming optical system;

FIG. 4 is a graphical representation of a divergence angle dependency ofthe intensity of laser light emitted from a semiconductor laser;

FIG. 5 is a view showing the optical system of FIG. 1 in which laserbeams are geometrical-optically converted into parallel laser beams;

FIG. 6 is a view showing the parallel laser beams exhibiting anwave-optically diverging nature;

FIG. 7 is a view showing the optical system of FIG. 5 in which one ofthe laser elements of the laser light source is displaced from theproper place;

FIG. 8 is a view showing the optical system of FIG. 5 in which a laserarray and a lens array are respectively used for the laser light sourceand the lens, and the laser array and the lens array are relativelydisplaced from each other;

FIG. 9 is a view showing the optical system of FIG. 5 in which theimagery spots are made insufficiently close to each other whenconsidering it from the electrophotographic view point;

FIG. 10 is a perspective view showing the construction of an embodimentof an information recording device according to the present invention;

FIG. 11 is a development showing an optical system upstream of a polygonscanner of the information recording device of FIG. 10 as viewed in theplane, which is orthogonal to the plane of deflection and includes anoptical axis;

FIG. 12 is a development showing the optical system downstream of thepolygon scanner of the information recording device of FIG. 10 as viewedin the plane, which is orthogonal to the plane of deflection andincludes an optical axis;

FIG. 13 is a development showing the optical system upstream of thepolygon scanner of the information recording device of FIG. 10 as viewedin the plane, which is in parallel to the plane of deflection andincludes an optical axis;

FIG. 14 is a development showing the optical system downstream of thepolygon scanner of the information recording device of FIG. 10 as viewedin the plane, which is in parallel to the plane of deflection andincludes an optical axis;

FIG. 15 is a perspective view showing a cylindrical lens array of thegradient index (GRIN) type, which is formed by diffusing ions into aglass substrate;

FIG. 16 is a perspective view showing a cylindrical lens array of theFresnel lens type, which is formed on a glass substrate;

FIG. 17 is a perspective view showing a cylindrical lens array made ofglass or plastic, which is formed by an injection molding method;

FIG. 18 is a perspective view showing a multibeam semiconductor laserarray in which a laser array and a microlens array are fabricated into asingle piece;

FIG. 19 is an explanatory diagram showing the principle of an interlacedscan in which a relationship between the imagery spots by laser beams onthe image plane and scan lines are illustrated;

FIG. 20 is a graphical representation of a variation of light intensityof the imagery spot with respect to the distance from the center of thespot to the circumference;

FIG. 21(a) is a view showing the basic construction of anotherembodiment of the present invention;

FIG. 21(b) is a graphical representation of a variation of the amplitudeof laser beam over line A-A' in FIG. 21(a);

FIG. 21(c) is a graphical representation of a variation of the intensityof laser beam over line B-B' in FIG. 21(a);

FIG. 22(a) is a view showing a modification of the system of FIG. 21(a);

FIG. 22(b) is a graphical representation of a variation of the amplitudeof laser beam over line A-A' in FIG. 22(a);

FIG. 22(c) is a graphical representation of a variation of the intensityof laser beam over line B-B' in FIG. 22(a);

FIG. 23(a) is a graphical representation of a variation of the intensityof a laser beam spot over the image forming surface in a normal imageforming optical system;

FIG. 23(b) is a graphical representation of a variation of the intensityof a laser beam spot over the image forming surface in the opticalsystem of FIG. 22(a);

FIG. 24 is a view showing a data reading system in an optical diskdevice incorporating the present invention, the data reading systembeing developed along a plane including the optical axis;

FIG. 25(a) is a view showing the relationship of a record pit formed ina track of a recording medium and a reading beam spot;

FIG. 25(b) is a view showing the positional relationship between anaperture and the reading beam reflected from the recording medium;

FIG. 25(c) is a view showing a profile of the read beam across theaperture;

FIG. 25(d) is a cross sectional view of the combination of a polarizer,aperture and light sensing element;

FIG. 26 is a view showing the basic construction of a multibeam opticaldisk device;

FIGS. 27(a) through 27(c) are views showing a case where the presentinvention is applied to the multibeam optical disk device of FIG. 26;

FIG. 28 is a view showing an optical system of the optical disk deviceaccording to the embodiment of the present invention, the optical systembeing developed along a plane including the optical axis;

FIG. 29 is a perspective view showing the construction of asemiconductor laser array used in the optical disk device of FIG. 28;

FIG. 30(a) is a plan view showing a micro-collimator lens array used inthe optical disk device of FIG. 28;

FIG. 30(b) is a sectional view showing the micro-collimator lens array;

FIG. 31 is a view showing a part of the optical system ranging from alight source to a beam expander, the partial optical system beingdeveloped along a plane including an optical axis;

FIG. 32 is a plan view showing an array of laser beam spots formed onthe recording surface of an optical disk in the embodiment of FIG. 28;

FIG. 33 is a timing chart showing an overwrite operation of the opticaldisk device of FIG. 28 when the recording medium is of themagneto-optical type;

FIG. 34 is a perspective view showing a device in which a semiconductorlaser array and a microlens array are formed into a single piece; and

FIG. 35 is a perspective view showing another device in which asemiconductor laser array and a microlens array are formed into a singlepiece.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described with reference to theaccompanying drawings.

In the present invention, optical means for reducing the divergence oflaser light is provided in connection with the laser light sources ofthe light source means.

With the provision of the optical means, the divergence angle of laserlight can be reduced, so that the distance between the imagery spotsformed on the image plane can effectively be reduced. This will bedescribed in detail.

FIG. 1 is an explanatory diagram showing the principle of the presentinvention. A plurality of (two in this instance) laser light sources LSemit laser light each at divergence angle θ₁. The laser light thenpasses through a lens L₂ which reduces the divergence angle of laserlight to θ₂. As a result, the laser light sources LS are equivalentlypositioned at an apparent light source plane P1 in FIG. 1 when opticallyviewed through an image forming lens L₁. With presence of the lens L₁,the light source plane P1 and an image forming plane P2 are opticallyconjugate. When lateral magnification β=f₂ /f₁, an imagery spot diameterof the laser beam on the image forming plane P2 is designated by d₂.Here, f₁ indicates a distance between the apparent light source plane P1and the lens L₁, and f₂ indicates a distance between the lens L₁ and theimage forming plane P2. In a case where the lens L₂ is not used as shownin FIG. 2, and β=f₂ /f₁, the diameter of an imagery spot of the laserbeam that is formed on the image forming plane P2 by the lens L₃, isdenoted as d₁. When comparing the above two optical systems, thedistance "r" between the laser light sources LS, that is, the distance"r" between the imagery spots, in one optical system is equal to that inthe other optical-system, but the spot diameter d₂ is larger than thespot diameter d₁. The reason why the spot diameter d₂ is larger than thespot diameter d₁ will be described below with reference to FIG. 3.

As known, the diameter d₀ of a beam spot on the image plane P3 is givenby

    d.sub.0 =4f.sub.2 λ/πD

where D: Beam diameter of laser beam incident on the image formingoptical system

λ: Wave length

f₂ : Focal distance of the image forming optical system

π: Circular constant

The d₀ is the optical spot diameter of which the circumference has alight amplitude 1/e (1/e² in power) as large as that at the spot center.The diameter D of the laser beam incident on the image forming opticalsystem is given by

    D=2f.sub.3 sin (θ.sub.1 /2)

where θ₁ : Divergence angle of laser light from the laser light sourceLS

f₃ : Focal length of a collimator lens L₄

The divergence angle θ₁ of the laser light is such an angle that whenthe light intensity on the principal line of the laser light isnormalized at 1, the light amplitude per unit angle is 1/e (1/e² inpower), as shown in FIG. 4. In the example shown in FIG. 4, asemiconductor laser array LSA in which the distance between the adjacentlight emitting elements is "r" is used for the laser light sources LS.In a case where the light source is a semiconductor laser, thedivergence angle of laser light is frequently expressed in terms of anangle (FWHM: Full Width at Half Mean) where the light intensity per unitangle on the principal laser light is halved (1/√2 in the lightamplitude). In using the divergence angle, this should be beared inmind.

When the lateral magnification β of the image forming system is f₂ /f₁,i.e., β=f₂ /f₁, in the subsidiary scan direction, the diameter of theimagery spot of the laser beam on the image plane P3 when viewed in thesubsidiary scan direction, is given by

    d.sub.0 =2λβ/πsin (θ.sub.1 /2)        (2)

The equation indicates that the diameter d₀ is uniquely determined bythe lateral magnification β and the divergence angle θ₁.

When the relation of equation (2) is applied to the optical systems ofFIGS. 1 and 2, the spot diameters d₁ and d₂ are expressed by

    d.sub.1 =2λβ/πsin (θ.sub.1 /2)

    d.sub.2 =2λβ/πsin (θ.sub.2 /2)

A relationship between the divergence angles θ₁ and θ₂, when α=a/b, maybe expressed by the following equation, using a formula of imageryrelation in the geometrical optics.

    θ.sub.2 =aθ.sub.1 /b=αθ.sub.1

where a: Distance between the laser light source LS and the lens L₂

b: Distance between the apparent light source plane P1 and the lens L₂

From the above equations, a relationship between the spot diameters d₁and d₂ may be expressed by

    d.sub.2 =αβd.sub.1                              (3)

The distance between two spots on the imagery plane is βr. Therefore, ifα is varied while β is fixed, the imagery spot diameter may be increasedwhile keeping the imagery spot distance on the imagery plane at a fixedvalue. If by utilizing this, the lateral magnification β of the imageryoptical system is properly adjusted to gain a desired line-to-linedistance, the spot-to-spot distance is effectively reduced. Thus, in thepresent invention, the spot diameter and the spot-to-spot distance onthe imagery plane can be changed by properly adjusting α and β.

Also in the present invention, the optical means for reducing thedivergence of laser light is a collimator optical system for arrangingthe laser beams into substantially parallel beams geometrical-optically,and the principal plane of the collimator optical system on the imageside and the image-formed surface of the photoreceptor are disposed inan optically conjugate relationship in the image forming optical system.

The construction of the information recording device of the invention asjust mentioned can further reduce the divergence angle of the laserlight.

The construction of the information recording device may be depicted asshown in FIG. 5. The operation of the construction of the informationrecording device will be described. Let us consider a case where thelaser beams are collimated by the lenses L₂ in FIG. 1. In this case,when considering this from the point of view of geometrical optics, theapparent light source plane is located at infinity and θ₂ =0. When morestrictly examining it from the point of view of wave optics, thedivergence angle θ₂ should be a certain divergence angle θ₃, which isnot 0. In FIG. 5, f₀ is a focal length of the lens L₂.

In an optical system as shown in FIG. 6, the divergence angle θ₃ isgiven by

    θ.sub.3 =2 sin.sup.-1 (2λ/πD.sub.1)        (4)

where D₁ : Beam diameter of the laser beam geometrical-opticallycollimated.

Equation (4) equivalently indicates that the laser light source of thedivergence angle θ₃ is located at the position of a collimator lens L₄.Accordingly, as shown in FIG. 5, the principal plane (apparent lightsource plane) P1 of the collimator lens L₂ on the image side and theimage plane (image forming plane) P2 are optically conjugate. Under thiscondition, the diameter d₃ of an imagery spot on the image forming planeP2 is given by

    d.sub.3 =2λβ/π sin (θ.sub.3 /2)       (5)

In the above equation, β=f₂ /f₁.

Substituting equation (4) into equation (5), then we have

    d.sub.3 =βD.sub.1                                     (6)

The distance r_(i) of the imagery spots on the scanned plane (imageforming plane) P2 is given by the following equation

    r.sub.i =βr                                           (7)

In the above calculations, the truncation effect of the laser beams bythe lens L₂ are not taken into account. In a case where the effectivediameter of the lens L₂ is finite, equation (4) can be rewritten into

    θ.sub.3 =2 sin.sup.-1 (2λ/πD.sub.1)A       (8)

In the above equation, A represents an apodization constant. Typicalvalues of the apodization constant A are as follows: A=1.39 when thelaser light is employed at the diameter where the power of the laserlight is 1/e² as great as that at the center, and A=1.97 when the laserlight is employed at the diameter where the power of the laser light is1/2 as great as that at the center. When the effective diameter of thelens L₂ is infinite, A=1. Such a phenomenon is called an apodization ofthe laser light. When considering the phenomenon, the portion in anoptical system where the diffraction is the greatest must bescrutinized. In a case where as in the first embodiment of the inventionto be given later, the lens L₂ is followed by a smaller stop, adivergence angle is first calculated using equation (4), and then anapodization for the smaller stop must be calculated.

In the information recording device of the invention, a laser arraytaking the form of a monolithic or single piece is used as the lightsource. A microlens array taking the form of a monolithic or singlepiece is used as the lens L₂ shown in FIG. 1 or 5. The constructionmakes it easy to realize the information recording apparatus having theconstruction as just mentioned. The term "microlens array" means a lensarray consisting of a group of miniature lenses each having the diameterin the range of approximately 10 μm to several mm.

An alignment precision of a lens will be described when the microlensarray as a single piece is not used. It is assumed that one of the laserlight sources LS, as shown in FIG. 7, is displaced by distance δ_(o)from the proper place in the direction vertical to the optical axis. Inthis case, the imagery spot is displaced from the normal place on theimage plane P2 by distance δ₁ as given by the following equation.

    δ.sub.1 =βδ.sub.o                         (9)

Considering a case where the optical system as shown in FIG. 5 is usedfor an actual laser beam printer, β≈1/16 as will be described in thefirst embodiment to be given later. In a laser beam printer of 800 spi(800 spots per inch) in resolution, the electrophotographic spotdiameter d_(x) is approximately 30 μm. When involving the spot diametercorrection coefficient "k", the optical spot diameter d₀ on the imageplane P2 is approximately 50 μm. A tolerable error of the position ofthe imagery spot on the image plane P2 is approximately 5% of theelectrophotographic spot diameter. Then, δ₁ ≈1.5 μm, and hence δ^(o) =24μm.

The microlens is a miniature lens of 0.1 to several mm in diameter, asstated above. A minimum diameter of the microlens that can bemanufactured by using the technique at the present stage is severalhundreds μm. If the distance between the microlenses is 500 μm, themanufacturing precision is in the order of δ_(o) /r=4.8×10⁻².Practically, it is difficult to gain this figure by arraying andmechanically adjusting the individual microlenses. The same thing isapplicable for the laser light source. However, when the laser arraysand the microlens arrays are fabricated using photolithography, theprecision in this order can easily be achieved. The dimensionalprecision of the photo-mask which the present technique can achieve isat least 10⁻⁶. In the microlens array made of plastic or glass that isfabricated by an injection molding method, the precision of 4.8×10⁻² canbe achieved.

Let us consider an alignment precision of the laser array and themicrolens array when those are used. In the description, it is assumedthat those arrays have no dimensional error, and that as shown in FIG.8, a semiconductor laser array LSA and a microlens array PMA are bothdisplaced from the proper place by distance δ_(a). As seen also fromFIG. 7 already referred to, the two imagery spots are displaced in thesame direction and by the same amounts of space, δ_(b) =βδ_(a). However,in the multibeam laser beam printer, such an absolute displacement isnot problematic in practical use. What is essential to this type of theprinter is the relative positional relationship between the two spots.

Another construction of the invention further improves the alignmentadjustment of the single piece laser array and the single piecemicrolens array. The space between the imagery spots on the image planemust be reduced in only the array direction of the laser array.Therefore, reduction of the divergence angle of the laser light only inthe array direction of the laser array suffices. To this end, the laserarray is constructed such that it consists of a cylindrical lens array,and the direction in which the cylindrical lens array exhibits a lensfunction is substantially coincident with the array direction of thelaser array. With such a construction, only the divergence angle of thelaser light in the array direction of the laser array can be reduced. Inthis case, displacement of the microlens array or the laser array in thedirection in which the cylindrical lens array does not exhibit a lensfunction, will have no effect on the image formation of the laser beam.Therefore, poor alignment precision in this direction is allowed.

Further, in the invention, a semiconductor laser array is formed on asingle substrate by photolithography process of high precision.Therefore, the precision, δ_(o) /r=4.8×10⁻², can be realized; which isrequired for the element-to-element distance in the laser array.

In the invention, the microlens array is a cylindrical lens arraydisposed such that the direction in which the cylindrical lens arrayexhibits a lens function is substantially coincident with the arraydirection of the laser array.

This construction can narrow the space between the imagery spots on theimage plane satisfactorily in practical use while minimizing the loss ofthe light quantity. In the electrophotographic process, the optimumvalue of the spot diameter correction coefficient "k" is approximately1.5, k≈1.5. As seen from equation (1), the electrophotographic spotdiameter d_(x) is smaller than the optical spot diameter d₀. Rearrangingequations (6) and (7), we have d₃ /r_(i) =D₁ /r. Thence, even if theend-to-end distance between the paired lens elements of the lens L₂, asshown in FIG. 9, is reduced to as short as possible, it is impossible tomake the imagery spots satisfactorily close to each otherelectrophotographically. That is, since the diameter of the lens issubstantially equal to the optical spot diameter, it is impossible tomake the imagery spots satisfactorily close to each otherelectrophotographically unless the two lens elements overlap.

One of the approaches to solve the problem is provision of an opticalstop at a proper location as in the first embodiment to be given later.This approach suffers from large loss of light quantity, though.Application of interlaced scanning to the information recording devicethus constructed can prevent such loss of light quantity.

Examples of the information recording device based on the technicalideas mentioned above will be described.

FIG. 10 is a perspective view showing the construction of an informationrecording device according to the first embodiment of the invention.FIGS. 11 and 12 are developments showing an optical system of theinformation recording device of FIG. 10 as viewed in the plane, which isorthogonal to the plane of deflection and includes an optical axis.FIGS. 13 and 14 are developments showing the optical system as viewed inthe plane, which is in parallel to the plane of deflection and includesthe optical axis.

A multibeam semiconductor laser array 1 is made up of two semiconductorlaser elements 1a, spaced by 500 μm, which are arrayed on a singlesubstrate and may be independently driven. Those laser elements 1a arepackaged into a single unit. The laser elements 1a are arrayed in thedirection orthogonal to the main scan direction, viz., in the subsidiaryscan direction. The junction surface between the laser elements 1aextends in the direction parallel to the array direction of the laserelements 1a. The laser elements 1a are of AlGaInP type and are eachcapable of emitting a beam of laser light of 680 nm in wavelength. Adivergence angle of laser light of each laser element is: θ^(a) =12°with respect to the direction parallel to the junction surface betweenthe laser elements 1a, and θ_(b) =50° with respect to the directionvertical to the junction surface between the laser elements. Thedivergence angle, as described above, is defined as an angle to satisfythe power relation 1/e².

Laser light emitted from the multibeam semiconductor laser array 1passes through a planer microlens array 2 and is collimated in thedirection parallel to the junction surface between the laser elements(see FIG. 11). The microlens array 2 is a gradient index (GRIN) lensfunctioning as a cylindrical lens of the convex lens. In the planeparallel to the junction surface, the GRIN lens exhibits its lensfunction. An example of the construction of the microlens array 2 isshown in FIG. 15.

As shown, the microlens array 2 is constructed such that two cylindricallenses 2b of the GRIN lens type are formed on the surface of a glasssubstrate 2a by diffusing thallium ions, for example. In the figure,reference numeral 2c indicates ion diffused regions. To form thediffused regions, a mask is formed over the glass substrate 2a byphotolithography. The glass substrate 2a with the mask is immersed infused salt containing thallium ions and the like, to diffuse thalliumions into the glass substrate. A portion of the glass substrate where adensity of thallium ions is high exhibits a high refractive index. Thecylindrical lenses 2b of the GRIN lens type are based on this nature. Inaddition, the method of manufacturing the planer microlens array 2 iswell known. For the manufacturing method, reference is made to "N.Yamamoto and T. Yamasaki, National Convention Record of IEICE (Instituteof Electronic information and Communication Engineers), 1083 (1983)".

The microlens array 2 is specified as follows: focal distance is 2000 μm(2.0 mm), NA (numerical aperture) is approximately 0.1, and effectivediameter of the lens as viewed in the direction in which it functions asa lens is approximately 400 μm (0.4 mm). This corresponds to a casewhere laser light is condensed to a place where its power is 1/e² highas the power at the center of the laser beam, that is, a case whereA=1.39 in equation (8). The distance between the two cylindrical lenses2b is 500 μm, which is equal to the distance between the laser elements1a.

Laser light leaving the microlens array 2 is collimated, by acylindrical lens 3 as a convex lens, in the direction vertical to thejunction surface between the laser elements 1a (see FIG. 13). Next, thelaser light is expanded by a cylindrical lens 4 of a concave lens, andthen is collimated again by a cylindrical lens 5. The collimated laserlight is regulated by an optical stop 6, and is imaged on a mirror 8a ofa polygon mirror 8 by a cylindrical lens 7 as a convex lens, only in thedirection parallel to the junction surface of the laser elements 1a (seeFIG. 11). The laser beams are parallel to each other in the directionvertical to the junction surface between the laser elements 1a. Thelaser light deflected by the polygon mirror 8 is focused on the surfaceof a photoreceptor 11 through an f-θ lens 9 and a cylindrical lens 10 asa convex lens. The cylindrical lenses 7 and 10 cooperate to form aso-called anamorphic optical system, which corrects the adverse effectby an inclination of the polygon mirror 8. The direction in which thepolygon mirror 8 scans coincides with the main scan direction. In thepresent embodiment, the direction vertical to the junction surfacebetween the laser elements 1a corresponds to the main scan direction.The direction 12 in which the photoreceptor 11 revolves coincides withthe subsidiary scan direction, and corresponds to the direction parallelto the junction surface between the laser elements. Optical images I₁and I₂ depicted on the photoreceptor 11 are developed by the generalelectrophotographic process, not shown.

Let us consider a laser beam printer of 800 spi (800 spots per inch) inresolution. The electrophotographic spot diameter is 32 μm (=25400μm/800 lines), and hence the pitch in the subsidiary scan direction isalso 32 μm. If the spot diameter correction coefficient "k"=1.5, thenecessary optical spot diameter is 48 μm. The principal plane of themicrolens array 2 and the surface of the photoreceptor 11 are disposedin an optically conjugate relationship. Since the pitch of the lenses ofthe microlens array 2 is converted into the pitch in the subsidiary scandirection, its lateral magnification β is: β=32 μm/500 μm=0.064 (≈1/16).The aperture of the optical stop 6 is selected so that the power of thelaser light is 1/2 the power at the beam center in both the main andsubsidiary scan directions. A quantity of the laser light is reduced tobe half of that when the optical stop 6 is not used. When the opticalstop 6 is not used, the optical spot diameter in the subsidiary scandirection of the imagery spot formed on the surface of the photoreceptor11 is approximately 24 μm. According to the truncation effect, theoptical spot diameter becomes 48 μm. The reason for this is thatsubstituting equation (8) into equation (5), we have d₃ =AβD₁, and byregulating the laser light to 1/2 in light quantity, A=1.97 and hence d₃is also doubled. The diameter of the imagery spot on the surface of thephotoreceptor 11 is approximately 24 μm long in the main scan direction.

A second embodiment of the information recording device according to thepresent invention will be described. The second embodiment is based onthe interlaced scanning system.

In the construction shown in FIG. 10, some of the parameters of theoptical system are altered and the interlaced scan shown in FIG. 19 isalso used. The distance between the two laser elements 1a of themultibeam semiconductor laser array 1 is 1000 μm (1 mm), the distancebetween the two cylindrical lenses 2b of the microlens array 2 is also1000 μm (1 mm), and the lateral magnification β=0.093 (≈=1/11). Theaperture of the optical stop 6 is set to be large, and the optical spotdiameter is approximately 48 μm. Under this condition, the laser lightpassing through the microlens array 2 is straightforwardly imaged on thephotoreceptor surface. Under this condition, the optical stop 6functions only to remove stray light, and causes little loss of light.The remaining parameters are the same as those in the first embodiment.

With the above construction, the electrophotographic spot diameter ofthe imagery spot formed on the surface of the photoreceptor 11 isapproximately 32 μm long in the subsidiary scan direction, and thedistance of the two imagery spots is approximately 96 μm. Therefore, therequirements for the interlaced scan in FIG. 19 are satisfied.

The embodiment shown in FIG. 10 uses the plane microlens array 2constructed with the cylindrical lenses of the GRIN type that are formedby an ion diffusion process. In place of the microlens array, acylindrical lens array as shown in FIG. 16 may be used. In thecylindrical lens array, a pair of cylindrical lenses 2d of the Fresnellens type are formed on a glass substrate 2a by electron beamlithography. The micro-Fresnel lens for the collimator of thesemiconductor laser device may be manufactured by using the technique asdescribed in "Low Aberration Visible Micro-Collimated Laser Diode", byM. Yoneda et al., The 1990 IEICE (Institute of Electronics-Informationand Communication Engineers) Spring National Convention Record, C-265(1990).

Further, the microlens array 2 used in the embodiment of FIG. 10 may bereplaced by a cylindrical lens array 2g as shown in FIG. 17. In thecylindrical lens array, a pair of cylindrical lenses 2f are formed byinjection molding a transparent material 2e of plastic or glass.

The interlaced scanning system may be used for the cases where thecylindrical lens array shown in FIG. 16 and the injection moldedcylindrical lens array shown in FIG. 17 are used in place of themicrolens array 2 of FIG. 10.

A multibeam semiconductor laser array as shown in FIG. 18 in which alaser array and a microlens array are fabricated into a single unit maybe substituted for the combination of the multibeam semiconductor laserarray 1 and the microlens array 2 in FIG. 10. In the multibeamsemiconductor array of FIG. 18, cylindrical lenses 22a to 22d arerespectively disposed in connection with semiconductor laser elements21a to 21d. The cylindrical lenses 22a to 22d are formed in a mannerthat a substrate 20 made of AlGaAs or AlGaInP are worked into lenses bydry-etching process. The structure of the semiconductor laser array andthe method of manufacturing the same are known, and for the details ofthem, reference is made to "Laser Diode Integrated with Microlens (II)"by J. Shimada and O. Ohguchi, in The 1991 IEICE (institute ofElectronics-Information and Communication Engineers) Spring NationalConvention Record, C-251 (1991). SiO₂, plastics or the like may also beused for the cylindrical lenses 22a to 22d. When using any of thosematerials, the end face 23 of the multibeam semiconductor laser arrayand the terrace 24 are formed by a dry-etching process. A film made of amaterial, such as SiO₂, plastics or the like, is formed thereover by asuitable method, such as sputtering or casting process, and then isselectively etched away to form the cylindrical lenses 22a to 22d.

Thus, the semiconductor laser elements 21a to 21d and the cylindricallenses 22a to 22d respectively associated therewith are formed on thesingle substrate in an integral form. Therefore, the number of steps toassemble the information recording device is reduced, thereby to providean easy manufacturing of the recording device. Additionally, there is noneed for optically aligning the multibeam semiconductor laser array withthe microlens array. The number of steps for the related adjustment isreduced.

Where the interlaced scan as mentioned above is employed, a highmechanical precision is required for the scan optical system, becausethe spot-to-spot distance on the image plane is relatively wide. Theconstruction of FIG. 18 is suitable for the interlaced scan, because itprovides the improved position precision of the laser elements withrespect to the cylindrical lenses.

The information recording devices according to the embodiments thus farmade are based on the electrophotographic process; however, if required,the present invention may be applied to a recording system that is basedon any of other processes than the electrophotographic process. Thepresent invention may be applied to a recording system in which aphotosensitive film, not the electrophotographic photoreceptor, isscanned, a thermal recording system based on the thermal effect of thelaser beams, an opto-magnetic recording system, and the like. In thoserecording systems, the optimum value of "k" in equation (1) is notalways equal to that in the recording system based on theelectrophotographic process. Therefore, the distance of the adjacentlaser light sources must be empirically determined depending on therecording process used.

The present invention may also be applied to an information recording orstorage system, such as a multitrack recording system in an opticalmemory.

Next, an optical disk device to which the present invention is applied,as another embodiment, will be described hereinafter.

FIG. 21(a) is a view showing the basic construction of this embodiment.A laser beam from a laser light source 101 is arranged into a parallellight beam by a collimator lens 102. The collimator lens 102 is a planarmicrolens of the distributed refractive index type, that is formed byion diffusion process. A phase shifter 103 as phase modulating means isprovided at the central part of the collimator lens 102. The phaseshifter 103 consists of a thin film that is made of SiO₂, Si₃ N₄ or thelike and of λ/2n thick where λ denotes the wavelength of a laser beam,and n denotes the refractive index of the thin film.

With use of the phase shifter 103, there is caused an optical pathdifference of λ/2 between the light beam passing through the centralpart of the collimator lens 102 and the light beam passing through theperipheral part of the collimator lens. FIG. 21(b) is a graphicalrepresentation of a variation of the amplitude of laser light over lineA-A' in FIG. 21(a). A plane P1 of the collimator lens 102 with the phaseshifter 103 formed thereon is optically conjugate to an image formingplane P2 with respect to an image forming lens 104. In other words, animage on the plane P1 is projected on the plane P2. FIG. 21(c) is agraphical representation of a profile of a light spot formed on theimage forming plane P2.

The amplitude variations of the light inside and outside the phaseshifter 103 are out of phase with respect to the peripheral edge of thephase shifter 103. Therefore, when an image on the plane P1 is formed onthe image forming plane P2, which is conjugate to the plane P1, theintensity of light at the peripheral edge of the phase shifter 103 issubstantially zero (0). As a result, the profile of the light beam spotis narrow in shape. Thus, the beam spot formed by the optical systemincluding the phase shifter 103 is narrower than that by the opticalsystem not including the phase shifter, while at the same time a sidelobe appears. The beneficial effects of the optical system abovedescribed can be achieved by causing a phase difference between thelight beam passing through the central part and the light beam passingthrough the peripheral part by some means.

A modification of the system of FIG. 21(a) is shown in FIG. 22(a). Inthis optical system, the diameter (e.g., 0.1 mm) of the phase shifter103 is smaller than that (e.g., 0.2 mm) of the phase shifter 103 in theoptical system of FIG. 21(a). With the reduction of the phase shifterdiameter;, the diameter of the beam spot formed can be further reduced.The amplitude of light varies on the plane P1 as shown in FIG. 22(b).The intensity of light is profiled on the image forming plane P2 asshown in FIG. 22(c).

A specific example of the profile of the laser spot formed by theoptical system of FIG. 22(a) is shown in FIG. 23(b). A profile shown inFIG. 23(a) is formed by an optical system of the same type, which doesnot include the phase shifter 103. In the figures, FWHM indicates alevel defining the Full Width at Half Mean. 1/e² indicates a leveldefining the diameter of the Gaussian beam profile. In the opticalsystem using the phase shifter 103, the diameter (0.4 μm) of the mainpeak at the central part is half of the diameter (0.8 μm) in the opticalsystem not using the phase shifter 103. However, a large side lobe,shaped like a doughnut, surrounds the main peak. A light diminishingmeans may be used in combination with the phase shifter 103. In thiscase, there is no need of reducing the diameter of the phase shifter103.

In the optical systems of FIGS. 21(a) and 22(a), the surface of thecollimator lens 102 having the phase shifter 103 formed thereon isdirected toward the image forming lens 104. If required, it may bedirected toward the laser light source 101. The intended object when thephase shifter is used can be achieved by placing the phase shifter 103close to the principal plane of the collimator lens 102. In a case wherethe thickness of the glass substrate of the collimator lens 102 cannotbe reduced, it is preferable to direct the phase-shifter bearing surfaceof the collimator lens 102 toward the laser light source 101 to gain aneasy assembling of the actual optical system. If the collimator lens 102has a thick glass substrate, the focal point of the collimator lens 102is positioned within the glass substrate.

In the present embodiment, data is written using the laser spot profiledas shown in FIG. 21(c), and is read out using the laser spot profiled asshown in FIG. 22(c). If the size of the phase shifter 103 is properlyselected, the diameter of the formed spot can be reduced byapproximately 20%, with a relatively small side lobe formed. Thecombination of the formed spot and the laser power control enables asmall pit to be recorded. For reading out data, the laser beam spot asshown in FIG. 22(c) is used, and the side lobe is removed by anaperture.

FIG. 24 shows a schematic illustration of an optical disk device whichallows high resolution data reading, using a laser beam spot as shown inFIG. 22. In the figure, like reference numerals are used for designatinglike or equivalent portions in the optical system of FIG. 21(a) and FIG.22(a).

A laser beam emitted from the laser light source 101 is arranged into aparallel light beam when considered from the standpoint of geometricoptics. The phase shifter 103 causes the optical path difference of λ/2between the light beam passing through the central part of thecollimator lens 102 and the light beam passing through the peripheralpart of the lens. The light beam emanating from the collimator lens 102is collimated by a collimator lens 104a, passes through a beam splitter105, and is focused on the image forming plane P2 by means of an imageforming lens 104b.

In a record mode, the phase shifter 103 of the large diameter shown inFIG. 21(a) is used. The Full Width at Half Maximum (FWHM) of the beamspot on the image forming plane P2 is approximately 0.8 μm as shown inFIG. 23(a). The diameter of a pit to be actually written into theoptical disk is approximately 0.4 μm.

In a reproduction mode, the phase shifter 103 of the small diametershown in FIG. 22(a) is used. The FWHM of the beam spot on the imageforming plane P2 is approximately 0.4 μm as shown in FIG. 23(b).Recorded data can be read out with the width of 0.4 μm. Since thedoughnut-like side lobe is present around the main peak, the reproduceddata includes not only the intended recorded data but also the recordeddata preceding and subsequent to the intended data and data recorded inthe adjacent tracks. To remove the side lobe, an aperture is coupledwith the light sensing portion as described below.

The light beam reflected from the recording medium passes through theimage forming lens 104b, is bent in its path by 90° by the beam splitter105, passes through an image forming lens 104c and an aperture 106, andreaches the light sensing surface of a light sensing element 107. Theaperture 106 is located at a position optically conjugate to therecording (image forming) plane P2. The focal distance of the imageforming lens 104c is approximately 100 times that of the image forminglens 104b. Accordingly, an image on the image forming plane P2 ismagnified 100 times and formed on the aperture 106. For simplicity ofillustration, the distances are not exact in the optical system shown inFIG. 24. In a case where the recording medium is of the magneto-opticaltype, a polarizer 108 is disposed in front of the aperture 106.

The principle of a reading operation in the optical disk deviceconstructed as shown in FIG. 24 is diagrammatically illustrated in FIG.25.

Record pits Rp formed in the track T on the recording medium and areading beam are illustrated in FIG. 25(a). The width of the main peakSc of the reading beam (see FIG. 25(c)) is selected to be substantiallyequal to that of the record pit Rp. In FIG. 25(a), shaded portions arethe areas of the reading beam where the light intensity is in excess ofthe FWHM level. The positional relationship of the aperture 106 and thereading beam reflected from the recording medium is shown in FIG. 25(b).The profile of the reading beam at the aperture 106 is shown in FIG.25(c). The cross section of the combination of the polarizer 108,aperture 106 and the light sensing element 107 is shown in FIG. 25(d).

The light beam reflected from the recording medium is imaged on theaperture 106 at the magnification of 100 times. The diameter Da of theaperture 106 is 80 μm in this instance. The value of 80 μm is equal tothe distance 100 times (corresponding to the optical magnification) thewidth of 0.8 μm of the profile at the level of zero shown in FIG. 23(b),which extends to both sides of the central peak.

The side lobe Ss of the reading beam is interrupted by the aperture 106,and only the main peak Sc is allowed to pass through the aperture 106.Therefore, a small record pit Rp can be read, and hence data can berecorded at a high recording density and the data thus recorded can beread out.

Thus, after the reflected light beam from the recording medium ismagnified, the side lobe is removed by the aperture 106. Accordingly,the diameter of the opening of the aperture 106 can be selected to belarge, approximately 80 μm.

In the optical disk device of FIG. 24, the polarizer 108 is disposed inproximity with the aperture 106. It is required when the recordingmedium is of the magneto-optical type, but is not required for therecording medium of the phase-change type or the read-only type.

An optical disk device using multiple laser beams, to which thehigh-density recording/reproducing technique as mentioned above isapplied, will be described hereinafter.

The basic construction of the multibeam optical disk device is shown inFIG. 26. Two collimator lenses 102 having the focal distance f₀ areprovided for laser light sources 101 and 101 in one-to-onecorrespondence manner. The laser beam emitted from the laser lightsource 101 is arranged into a parallel light beam by means of thecollimator lens 102 when considered from the standpoint of geometricoptics. However, when considered from the standpoint of wave optics, ithas a divergence angle θ₃. The divergence angle θ₃ is smaller than thedivergence angle θ₁ of the laser beam emitted from the laser lightsource 101. The laser beam is focused on the image forming plane P2 bymeans of the image forming lens 104, thereby forming a beam spotthereon. In the figure, f₁ indicates the distance between the plane P1of the collimator lens 102 and the image forming lens 104, f₂ indicatesthe distance between the image forming lens 104 and the image formingplane P2.

When the divergence angle of the laser light is decreased, the beam spotdiameter d₃ increases while the spot-to-spot distance r_(i) on the imageforming plane P2 remains unchanged. This fact implies that where thedistance r_(i) between the laser light sources 101 and 101 is large, thespot-to-spot distance r_(i) on the image forming plane P2 can besubstantially reduced by adjusting the magnification in the opticalsystem, if the divergence angle is small.

A schematic illustration of a multibeam optical disk device of FIG. 26to which the present invention is applied is shown in FIG. 27. Laserbeams emitted from laser sources 101₁ to 101₃ pass through planarmicrolenses 109₁ to 109₃ as collimator lenses and phase shifters 103₁ to103₃, and are imaged with spots S₁ to S₃ on the image forming(recording) plane P2 by means of the image forming lens 104. The planeP1 of the planar microlenses 109₁ to 109₃ having phase shifters 103₁ to103₃ formed thereon is optically conjugate to the image forming plane P2with respect to the lens 104. The formed spots S₁ and S₂ have profilesas shown in FIG. 27(c) (corresponding to FIG. 21(c)). The spot S₃ has aprofile as shown in FIG. 27(b) (corresponding to FIG. 22(c)). Of thosespots S₁ to S₃ formed on the same track, the spot S₃ is used for theread operation, the S₂ for the erasure operation, and the spot S₁ forthe write operation.

The optical system of the multibeam optical disk device shown in FIG. 27is schematically illustrated in FIG. 28. An optical disk 111 is rotatedat a constant. Speed by a motor 112. An optical head consists of anoptical head fixed portion 113 and an optical head movable portion 114.Within the optical head fixed portion 113 a semiconductor laser array115 as a light source is disposed which is a two-dimensional array ofsemiconductor laser elements of the surface emission type. Laser beamsemitted from the two-dimensional semiconductor laser array 115 arecollimated by a micro-collimator lens array 116.

FIG. 29 is a perspective view showing the construction of thetwo-dimensional semiconductor laser array 115 used in the optical diskdevice of FIG. 28. As shown, a plurality of semiconductor laser elements115b are arrayed in a matrix fashion on a substrate 115a made of GaAs,for example. One side of each column of semiconductor laser elements115b is grooved in the column direction. The groove, which is slanted at45° to the substrate surface as viewed in cross section, has an endsurface 115c. The other side of each column of the semiconductor laserelements is also grooved in the column direction. The groove, which isvertical to the substrate surface as viewed in cross section, has an endsurface 115d serving as a mirror face. Each end surface 115c, slanted at45° to the substrate surface, serves as a total reflection prism. Eachsemiconductor laser element, when combined with the end surface 115d anda light emission surface 115e, forms a Fabry-Perot resonator. The endsurfaces 115c and 115d are formed by reactive dry etching process (RIE)of the Chlorine family. The semiconductor laser array thus structured isknown as disclosed by T. Takamori, L. A. Coldren, and J. L. Merz intheir paper: "Lasing Characteristics of a Continuous-Wave OperatedFolded-Cavity Surface Emitting Laser", Appl. Phys. Lett. 56(23),pp2267-2269 (1990). In the semiconductor laser array 115, the distancer₁ between the adjacent elements as vertically viewed is 100 μm, and thedistance r₂ between the adjacent columns of the elements is 300 μm.

The construction of the micro-collimator lens array 116 is illustratedin detail in FIGS. 30(a) and 30(b). As shown, planar micro-lenses 116bof the gradient index (GRIN) type are formed by diffusing metal ions,such as silver ions, thallium ions or the like, into the surface regionof a glass substrate 116a. The planar microlens thus structured is knownas disclosed by M. Oikawa, K. Iga, and T. Sanada in their paper:"Distributed-Index Planar Micro-Lens Prepared from Ion ExchangeTechnique", Jpn. Appl. Phys., 20(4), L296-L298(1981). Phase shifters116c and 116d as thin films made of SiO₂ or Si₃ N₄ are formed in thecentral parts of the planar microlenses 116b, respectively. Each phaseshifter functions to cause the optical path difference between the lightbeam passing through the center of the collimator lens and the lightbeam passing through the peripheral part of the same. The optical pathdifference is half of the wave length of the laser light. In thisinstance, the diameters of the phase shifters 116c and 116d aredifferent from each other. The phase shifter 116c of the small diameteris used for the reading operation, and the phase shifter 116d, for theerasing and writing operations. The element-to-element distance r₁ (asviewed horizontally) and the row-to-row distance r₂ in themicro-collimator lens array 116 must be selected to be equal to thecorresponding distances r₁ and r₂ in the semiconductor laser array 115.

Returning back to FIG. 28, laser beams emanating from themicro-collimator lens array 116 reach the optical head movable portion114 after passing through a beam splitter 118a and a beam expander 125.The beam expander 125 includes a concave lens 123 and a convex lens 124.The convex lens 124 is movable in the directions of arrow heads 129 bymeans of a driver (not shown).

The optical head movable portion 14 includes a prism 126 and a convexlens 127, and is movable bidirectionally as indicated by arrow heads 128by means of a drive means, not shown. The optical head movable portion114 further contains a coil 130 for applying a magnetic field. Amagnetic field modulated at fixed periods is applied to an optical disk111 when the data is erased or reproduced. In the optical head movableportion 114, the laser beams are bent by the prism 126 and then arefocused on the optical disk 111 by means of the convex lens 127. Theoptical head movable portion 114 is of the floating type.

The laser beams reflected on the surface of the optical disk 111 passthrough the optical head movable portion 114 and the beam expander 125,and reach the beam splitter 118a. The returned laser beams are reflectedtoward the beam splitter 118b. The beam splitter 118b splits each laserbeam into two beams. One of the beams passes through an aperture array117 with a polarizer and hits a photodiode array 119 where it isconverted into an electrical signal as a reproduced signal. Thephotodiode array 119 is a linear array of photodiodes, viz., consists ofphotodiodes linearly arrayed. The other laser beam emanating from thebeam splitter passes through a cylindrical lens 120 and a lens 121, andhits a quartered photodiode 122 which in turn produces an electricalsignal. The produced electrical signal is used for detecting a focuserror. A tracking error signal is also generated from the reproducedsignal output from the photodiode array 119. The focus error signaldetected by the quartered photodiode 122 is used for the focus control.

In the present embodiment, a plurality of laser beams are imaged atextremely close positions on the optical disk 11. Therefore, it isdifficult to pick up the focus error signal and the tracking errorsignal. To cope with this, a time point on which only one of the laserlight sources lights is made to recur in the record or reproductionmode, in order to pick up the focus error signal or the tracking errorsignal. A frequency of appearances of the time point must be selected soas not to interrupt the focus control and the tracking control.

A sectional optical system including the semiconductor laser array 115,micro-collimator lens array 116, beam splitter 118a, concave lens 123,and convex lens 124 is shown in FIG. 31. Since a laser beam emitted fromthe micro-collimator lens array 116 has a small divergence angle, thebeam diameter is expanded by the beam expander 125 including the concavelens 123 and the convex lens 124, thereby to obtain an intended beamdiameter.

The laser beam spots formed on the recording surface of the optical disk111 in the optical system of FIG. 28, is arrayed as shown in FIG. 32. Afirst group 141 of beam spots is used for reading data out of theoptical disk. A second group 142 of beam spots is used for erasing therecorded data. A third group of beam spots 143 is used for recordingdata. Thus, the first quartet of beam spots for data read, the secondquartet of beam spots for data erasure, and the third quartet of beamspots for data write are disposed in this order from upstream of themoving recording medium to downstream. In the first group 141 of beamspots, each spot has a profile as shown in FIG. 22(c). In the second andthird groups 142 and 143 of beam spots, each spot has a profile as shownin FIG. 21(c). In those groups 141 to 143 of beam spots, thespot-to-spot distance P_(S) is larger than the track pitch P_(T). Inthis instance, the distance P_(S) is three times the track pitch P_(T)since the beam spots for data read have large doughnut-shaped sidelobes. The recording tracks present between the adjacent spots are usedfor data record and read when another scan is performed in an interlacedscanning mode. For the details of the interlaced scanning, reference ismade to the paper written by T. Ota, M. Ito and S. Tatsuoka: "Spacing ofLaser diode array for Multi-beam Laser Printer using Interlacedscanning", Extended Abstract (Autumn meeting, 1991), The Japan Societyof Applied Physics, 11p-ZM-19 (1991).

In this instance, the track pitch P_(T) of the recording track 40 is 0.8μm. The element-to-element distance r₁ in the semiconductor laser array115 and the micro-collimator lens array 116 is 100 μm. The principalplane of the micro-collimator lens array 116 is projected onto therecording surface at the magnification β of 0.024 times (=0.8 μm×3/100μm). This relation is valid for the combination of the recording surfaceand the aperture array 117. Accordingly, apertures of 16 μm are linearlyarrayed at pitches of 100 μm in the aperture array 117.

FIG. 33 is a timing chart showing an overwrite operation of the opticaldisk device of FIG. 28 when the recording medium is of themagneto-optical type. An alternating current having a predeterminedperiod of time is fed to the coil 130 shown in FIG. 28. An alternatingmagnetic field developed by the coli is applied to the recording surfaceof the optical disk 111. A waveform of the magnetic field intensityshown in FIG. 33(a) represents a variation of the alternating magneticfield applied to the recording surface with respect to time. In thegraph of FIG. 33, the abscissa represents time. Further, the arrow headsindicate the directions of the magnetic field. The data erasure spots142a to 142d are lit up at the timings as shown in FIGS. 33(b) to 33(e).The spots 142a to 142d are simultaneously lit with a predeterminedperiod T. The magnetic materials in the recording surface of the diskpassing the erasure spot group 142 are unidirectionally magnetized,thereby erasing the data recorded therein. The record spot group 143 islit when the magnetic field is applied to the recording surface. In thiscase, the magnetic field applied is opposite in direction to thatapplied when the erasure spot group is lit. The record spots 143a to143d shown in FIGS. 33(f) to 33(i) are modulated by an external signal.Accordingly, data to be recorded is recorded on the disk. One period ofthe alternating magnetic field is selected so as to allow the recordingmedium to move by the distance substantially equal to the spot diameter.

While in the embodiments thus far described, the optical disk is of themagneto-optical type, it is evident that the present invention isapplicable for the optical disk of the phase change type. In thephase-change optical disk system, the coil 130 in FIG. 28 is notrequired. Data can be erased by anneal if the distance between the erasespot group 142 and the record spot group 143 is properly selected.

The optical head used in the embodiment is the floating optical head ofthe separation type. As a matter of course, the present invention isapplicable to the usual optical head. The invention must additionallyuse the beam expander, for example, that is not used in the normaloptical head. As the result of using the optical disk, the weight of thehead is increased and the access speed is slow in a random access mode.In the floating optical head, the additional optical system is installedin the fixed portion of the head, so that the access speed reductionproblem will not be created. In this respect, it is preferable to usethe floating optical head of the separation type when the presentinvention is carried out.

Some applications of the optical disk make much account of the datatransfer speed rather than the recording density. A multibeam lightsource in which a semiconductor laser array and a microlens array arefabricated into a single unit as shown in FIG. 34, is suitable for suchapplications. In the semiconductor laser array, a plurality ofsemiconductor laser elements are arrayed in the direction orthogonal tothe moving direction of the optical disk. In the microlens array, aplurality of microlenses are arrayed in a similar fashion.

To fabricate the semiconductor laser array structured as shown in FIG.34, a plurality of semiconductor laser elements 151a to 151d are formedon a substrate 150. The substrate 150 having the laser elements formedthereon is dry-etched so that the light emitting end faces of the laserelements 151a to 151d are exposed. As a result, a terrace 153 is formed.Then, SiOx, for example, is deposited over the terrace by the sputteringmethod. The structure is dry-etched to form cylindrical lenses 152a to152d, which are respectively associated with the laser elements 151a to151d.

The semiconductor laser array thus structured and the method offabricating the same have been known as described by J. Shimada, O.Ohguchi, and R. Sawada in their paper: "Microlens Fabricated by thePlanar Process", Journal of Lightwave Technology, Vol. 9, No. 5,pp571-576 (1991). In place of the microlenses 152a to 152d, adistributed index lens, which serves as a lens vertical to thesubstrate, may be formed by properly changing the composition ratio inthe film formed by depositing SiOx, for example.

The structure of the semiconductor laser array shown in FIG. 34 may bemodified into the structure as shown in FIG. 35. In the semiconductorlaser array of FIG. 35, microlenses 154a to 154d are formed such thatthe flat surfaces of the microlenses are directed toward the outside thesubstrate, as shown in FIG. 35. Phase shifters 155a to 155d arerespectively formed on the flat surfaces of the microlenses 154a to 154dby the combination of deposition process and lithography. In this case,the work of forming the phase shifters 155a to 155d is easy since theflat surfaces of the microlens, on which the phase shifters are formed,face outside the substrate.

In another modification, the basic structures of the semiconductor laserarray shown in FIG. 34 or 35 is employed. The number of semiconductorlaser elements is reduced to 3. The laser elements are arrayed in thesame direction as the moving direction of the optical disk, that is, inthe track extending direction. The laser beams emitted from therespective semiconductor laser elements can be used for data read,erase, and record. In other words, the usual over-write system can berealized.

While the two-dimensional semiconductor laser array structured as shownin FIG. 29 is used in the above-mentioned embodiment, a two-dimensionalsemiconductor laser array in which a diffraction grating or a TJSstructure is used in place of the reflecting mirror may be used.Further, instead of ion diffused microlens element, microfresnel lenselements may be used as microlens array 106.

It is evident that the optical disk device of the invention isapplicable for every type of recording medium, such as the recordingmedium of the RO, WORM, phase change, or magneto-optical type. When theinvention is applied for the optical disk of the magneto-optical type,the optical disk containing a single magneto-optical recording layer maybe used. The optical disk of the single layer structure can bemanufactured in a simpler manner than the optical disk of thedouble-layer structure.

What is claimed is:
 1. An optical disk device comprising:a disk-line recording medium; a drive mechanism for rotating said recording medium; and a movable optical head for recording and reproducing data into and from said recording medium with a laser beam, wherein said optical head comprises at least one light source section including a laser light source, a collimator lens for arranging a laser beam emitted from the laser light source into a parallel laser beam, an image forming lens for focusing the laser beam emanating from the collimator lens on the recording medium, and phase modulating means substantially located on a principal plane of said collimator lens, wherein said principal plane and the recording medium are disposed in an optically conjugate relationship with respect to said image forming lens.
 2. The optical disk device according to claim 1, wherein said phase modulating means causes a phase difference between a light beam passing through a central portion of the collimator lens and a light beam passing through the peripheral portion of the collimator lens.
 3. The optical disk device according to claim 2, wherein said phase modulating means includes a disk-like phase shifter located at the central portion of the collimator lens.
 4. The optical disk device according to claim 2, wherein said phase modulating means includes a ring-like phase shifter located at the peripheral portion of the collimator lens.
 5. The optical disk device according to claim 1, wherein said optical head further includes a reproducing optical system for focusing a reflected light beam from the recording medium on a light sensing element, and wherein an aperture is located at such a place where said aperture is close to the light sensing element and is optically conjugated to the recording surface of the recording medium.
 6. An optical disk device comprising:a disk-like recording medium; a drive mechanism for rotating said recording medium; and a movable optical head for recording and reproducing data into and from said recording medium with a laser beam; wherein said optical head includesa plurality of light source sections, each having a laser light source for emitting a laser beam, a plurality of collimator lenses, each corresponding to a respective one of said plurality of light source sections, for arranging the laser beams emitted from the respective light source section into parallel laser beams, an image forming lens for focusing the laser beans emanating from said plurality of collimator lenses on an image forming plane of said recording medium, and a plurality of phase modulating means substantially located on a principal plane of said plurality of collimator lenses, wherein said principal plane and said image forming plane are disposed in an optically conjugate relationship with respect to said image forming lens.
 7. The optical disk device of claim 6, wherein said plurality of phase modulating means have different sizes.
 8. An optical disk device comprising:a disk-like recording medium; a drive mechanism for rotating said recording medium; and an optical head for recording and reproducing data into and from said recording medium with a laser beam, wherein a fixed portion of said optical head includesa semiconductor laser array having a plurality of semiconductor laser elements on a first substrate for producing a plurality of laser beams; and a micro-collimator lens array for collimating the laser beans from said semiconductor laser array having a plurality of planar micro-lenses formed in the surface region of a second substrate and a plurality of phase shifters formed in a central part of a respective one of said plurality of planar micro-lenses, and wherein a movable portion of said optical head includes an image forming lens for focusing the laser beams emanating from said micro-collimator lens array on an image forming plane of said recording medium.
 9. The optical disk device of claim 8, wherein said plurality of phase shifters have different sizes. 